Parkinson's disease is a chronic, progressive neurodegenerative disease that generally develops after middle age. Initial symptoms include unilateral resting tremor, akinesia and rigidity. The tremors, akinesia, and rigidity are called the three major signs of Parkinson's disease, and each of them is caused by the selective death of dopaminergic neurons projected from the substantia nigra to the striatum. The etiology of the disease is still unknown; however, accumulated evidence suggests that an impaired energy-generating system accompanied by abnormal mitochondrial function of nigrostriatal dopaminergic neurons triggers the neurodegenerative disorder of the disease. The mitochondrial dysfunction has been assumed to subsequently cause oxidative stress and failure of calcium homeostasis, thereby resulting in neurodegeneration (Non-Patent Document 1).
Treatments of Parkinson's disease are roughly classified into medical management (medication) and surgical management (stereotaxic operation). Of these, medication is an established therapy and regarded as a basic treatment. In the medication, a symptomatic therapeutic agent is used to compensate for the nigrostriatal dopaminergic neuronal function denatured by Parkinson's disease. L-dopa exhibits the most remarkable therapeutic effects. It is said that no agent exceeds the effectiveness of L-dopa. Currently, L-dopa is used together with a dopa decarboxylase inhibitor to prevent the metabolism thereof in the periphery, and the desired clinical effects have been obtained.
However, L-dopa treatment has drawbacks in that, after several years of usage, there is a recurrence of movement disorders such as dyskinesia, and the sustainability and stability of the drug's effects are lost, resulting in fluctuations within each day. Moreover, side effects including digestive problems such as nausea and vomiting brought on by excessive release of dopamine, circulatory organ problems such as orthostatic hypotension, tachycardia and arrhythmia, and neurological manifestations such as hallucination, delusion and distraction have been a cause for concern.
Thus, in order to decrease the L-dopa preparation dosage and thereby reduce the side effects, multidrug therapies, in which dopamine receptor agonists, dopamine metabolism enzyme inhibitors, dopamine releasers, central anticholinergic agents and the like are used in combination, are employed. While such therapeutic advances remarkably improve prognoses, there is still no fundamental cure for Parkinson's disease and other neurodegenerative diseases. Medication must be taken for the rest of the patient's life, and the aforementioned drawbacks, i.e., decreased efficacy during long-term administration, side effects, and uncontrollable disease progression, can result from L-dopa monotherapy. In addition, it is difficult to expect dramatic effects, even with the employment of multidrug therapies.
Alzheimer's disease is a progressive neurodegenerative disease that affects various cognitive functions, primarily causing impairment of memory. Pathologically, Alzheimer's disease is characterized by the degeneration of synapses or neurons in the hippocampus and cerebral cortex, and the accumulation of two types of abnormal fibrils, i.e., senile plaques and changes in neurofibrils. Although the disease etiology is not completely understood, amyloid β protein (Aβ), which is derived from amyloid precursor protein (APP) by various mechanisms, is known to play an important role. Currently, cholinesterase inhibitors (tacrine, Aricept, rivastigmine, and galantamine) are used in the treatment of Alzheimer's disease for ameliorating symptoms, because acetylcholinergic nervous system in the brain is involved in cognitive function, and marked deficits in the acetylcholinergic system are observed in Alzheimer's disease. N-methyl-D-aspartate glutamate receptor antagonists (memantine) are also in practical use because hyperexcitability of the mechanism of glutamate neurotransmission is associated with neural degeneration or impairment. Neither monotherapy nor combination therapy using these drugs, however, has produced sufficient therapeutic effects, nor are they capable of halting the progression of the disease. Furthermore, gastrointestinal symptoms such as nausea and diarrhea are observed as side effects of cholinesterase.
With respect to ischemic neurodegenerative disorders induced by cerebral infarctions, such as atherothrombotic cerebral infarction, lacunar infarction, cardiogenic cerebral embolism, etc., the usage of very early thrombolytic therapy using tissue plasminogen activator (tPA) is rapidly increasing. This therapy, however, has many problems including a window as short as within three hours after the onset of disease, hemorrhagic complications, etc. In Japan, a free radical scavenger, edaravone, is used for a brain protection therapy. Although edaravone can be used concomitantly with tPA, sufficient clinical results have not been obtained.
Accordingly, there exists a strong need for a pharmaceutical agent having a novel mechanism of action, or a neuroprotectant for preventing neural degeneration or impairment from its etiologies such as abnormal mitochondrial function, etc.